When acquiring MR data it is relatively easy to control location, area dimensions and the thickness of each of the slices and to obtain data from the entire slice. (A slice as used herein includes "slab" which is defined as a slice with greater than normal thickness). However, when there is a particular section, such as a localized volume in a larger excited volume of interest (VOI) it is more difficult to obtain data from that localized volume and exclude data from the rest of the VOI. The capability of obtaining data from a particular localized volume within a larger excited volume is especially important for in-vivo spectroscopy. Such a capability will, for example, enable acquiring spectroscopic data exclusively from diseased portions of organs for comparison with data exclusively from healthy portions of the organs.
One of the prior art methods of volume localization uses surface coils. If the region of interest is a relatively small volume, a small coil has to be used to obtain localized data. However, small coils have limited penetration depths and thereby preclude obtaining data from volumes that are not superficial.
There are other prior art methods for selective volume excitation. See for example, a communication in the Journal of Magnetic Resonance, Volume 70, pages 488-492 (1986) entitled "Selected Volume Excitation Using Stimulated Echoes (VEST). Applications to Spatially Localized Spectroscopy and Imaging", written by the inventor herein. In the procedure explained in the communication, a volume of interest is excited using stimulated echoes with Gx, Gy and Gz gradients.
In the prior art it is known to obtain data localized to a given VOI; for example, in the form of a three dimensional solid bar by saturating the volumes surrounding the volume of interest. The prior art, however, does not teach an effective process and/or system for acquiring data from multiple localized volumes in the VOI. Nor does the prior art teach an effective method of acquiring multiple volume data from elements such as 31P in-vivo because of its short T2 relaxation time.
If a diseased tumor is located in the head, for example, it is preferable to use a multiple volume scan to acquire data from the tumor and also from normal tissue. To focus on the tumor to the exclusion of extraneous data, a localized VOI basically containing the tumor should be excited to the exclusion of surrounding volumes. The slabs of the excited volume are used to obtain the requisite data, for example, to compare the tumorous tissue with healthy tissue. Hence it is important to be able to acquire data from the a multiplicity of slabs in a selected locally excited VOI.
Accordingly, there is a need for improved magnetic resonance spatial localization to obtain data from multiple localized volumes in a volume of interest. The spatial localization could also be used for acquiring 31P spectroscopic data or for imaging.
Clinical and research applications of in-vivo nuclear magnetic resonance spectroscopy (MRS) necessitate well defined spatial localization of the target VOI. This necessity led in the past several years to the discovery of a multitude of methods for spatial localization. Of these methods only few are suitable for studies of 31P because of the added constraint of short time interval between spin excitation and signal acquisition when dealing with short T.sub.2 metabolites.
Accordingly, those skilled in the art are still searching for a spatial localization method which is applicable to both MRI and MRS to increase image resolution or to reduce scan times and to obtain data for 1-H or 31-P MRS.